I could never really explain why I didn't like Avalance but I'm gonna try and explain...
If I'm being honest, the only good thing about their relationship was Ava's growth cause it tied in the clone plot very well and made her less stiff. But I honestly don't think they should've lasted. And I mean this in the nicest way
Sara to me always belonged with Nyssa, sure they were both assassins but at the best point in their relationship they made each other better and grow as their own people. They stayed who they were but they grew together and helped each other.
With Avalance, at least to me, it seemed as if they were just in a standstill and never left the honeymoon phase. They spent so much time in that phase that they just became the same character just split into 2 people. Neither had a discernible personality that set them apart aside from their given quirks that they had when first introduced. Ava has her Organizational quirk and Sara her scarcely developed "personality", but in all honesty they feel the same to me.
Ava was is the character that I would think Sara would become of she never got on the gambit and followed in her dad's footsteps with law enforcement/government work.
(Also not to mention that they're technically"sisters" since the real Sara died and bishop created both Ava and Sara only using a small part of the human Sara's DNA)